The present invention relates to absorption spectroscopy systems and in particular to an absorption spectroscopy system having improved background light rejection.
Absorption spectroscopy is a tool of chemical analysis that measures light absorption by a material being investigated over a range of different frequencies. In laser absorption spectroscopy, a laser may be directed through the material, for example a gas within a test cell, while the laser frequency is tuned over the absorption range of the sample in a “sweep” of frequencies. The amplitudes of light before and after it passes through the material are compared to produce an absorption spectrum.
A practical problem in measuring absorption spectra using this technique is that the light detector receiving light from the test cell must be able to discriminate small intensity variations caused by absorption in the presence of the a strong “background” signal from the laser from the un-absorbed laser light striking the light detector.
Background laser signal may be suppressed to some extent through the use of modulation techniques such as “wavelength-modulation spectroscopy” (WMS) or “frequency-modulation spectroscopy” (FMS). In both techniques, the wavelength or frequency of the laser is “dithered” as it is swept through a range of frequencies, the dithering being a slight frequency modulation on top of monotonic modulation of the frequency sweep. Dithering serves to frequency encode the absorption bands in the material as a result of interaction of wavelength dithering with the absorption feature. This frequency encoded absorption signal may be de-modulated to eliminate the substantially unmodulated background radiation.
While this approach can greatly reduce the influence of background radiation, there are practical difficulties. For example, when a tunable diode laser used as the light source and the dithering is implemented by changing the injection current of the diode, a relatively small frequency range of dithering is possible; a range that may be much smaller than the absorption feature. Further, not all laser sources (for example, some lasers that may have desired frequencies for particular absorption features) permit injection current modulation or make the injection current accessible. Controlling the dithering through injection current control can be complex and it can be difficult to control the dithering in a stable manner or to manage incidental amplitude modulation that can obscure the desired signal.